bootloader & io.asm

;;;; bootloader.asm
;;;; Bootloader for the OS
org 0x7c00
bits 16

;;; Start/main function
start:
	jmp boot

;;; Includes
%include "io.asm"

;;; Boot function
boot:
	cli					; no interrupts
	cld					; only thing needed to init
	mov		dh, 5		; move decimal 5 into dh (row)
	mov		dl, 5		; move decimal 5 into dl (column)
	call	movCursor	; call movCursor
    call	print		; call putChar
	hlt					; halt the OS

;;; 512 available bytes, clear the rest with 0
times 510 - ($-$$) db 0
dw 0xAA55 					; Boot signature


;;;; io.asm
;;;; IO subroutines/functions for the OS
movCursor:
	; dh = Y coord, dl = X coord, bh = page #
	; void return
	mov 	ah, 02h 		; move 0x02 into ah
	mov 	bh, 0			; move decimal 0 into bh (page number)
	or 		dh, 5   		; if dh != 5
	jnz		incCursorPosH 	; jump to incCursorPosH
	int		0x10    		; call
	add 	dh, 1			; increase register dh by 1
	ret

incCursorPosH:
	; dh = row #
	; void return
	add		dh, 1		; increase register dh by one
	int		0x10		; call
	ret

incCursorPosV:
	; not needed right now
	; dl = column #
	; void return
	add		dl, 1		; increase register dl by one
	int 	0x10		; call
	ret

putChar:
	; al = char, bl = text color, cx = # of times printed
	; bh = page number
	; void return
	lodsb 				; load a byte from si
	or		al, al		; OR to check al
	jz		done		; if 0, there's nothingo left for the string, so end
	mov		ah, 0eh		; move 0x0a into ah
	mov		bh, 0		; move decimal to page number of register bh
	mov		bl, 4		; move decimal 4 to text color of register bl
	mov 	cx, 1		; move decimal 1 into cx (print character once)
	int 	10h			; call
	call	movCursor 	; call movCursor and go the next position
	jmp	 	putChar		; loop back & restart putChar

done:
	ret

print:
	; ds:si = null-terminated string
	; void return
	mov		ax, 0x07c0		; move 0x07c0 into ax
	mov		ds, ax			; move that segment into ds (segment)
	mov		si, message		; move the message into si (offset)
	call 	putChar			; call putChar
	call 	movCursor		; call movCursor

;;; Variables
message db 'Hello World', 0